Methods of underwater welding known in Japan include a dry method in which a chamber from which water has been removed is provided in the water and welding is carried out in this chamber, and a wet method in which welding is carried out directly in the water. As the wet method, heretofore, there has been proposed in Japan a method in which a gas is injected from the outer periphery of the tip end of a welding rod which arc welding is carried out within the atmosphere of the injected gas (See Japanese Patent Publication No. 45-32139), and a method in which shielding is effected with viscous liquid and plasma welding is carried out within the shielding.
As a consequence of recent progress in ocean cultivation, field welding works for large-scale marine structures are needed, and so a need has arisen for an underwater welding technique. Except for the dry method, underwater welding has seldom been employed in practice. However, the wet method is preferable for economic reasons and especially for welding a complex joint structure, and thus its practical development has been desired. While some research on the wet method has been carried out on a laboratory scale in Japan, degradation of the welding characteristics cannot be obviated because the above mentioned method involves welding within a water atmosphere, and so the wet method has not yet reached the stage of practical success.